(DISCLAIMER; I am never saying they are the best, just my favorite)
Worthy of note, but not my favorite.
--Dio N'arap - Sopor Aeternus and The Ensemble of Shadows
Not so much of a cover but an adaptation based on Black Sabbath's Paranoid, changed lyrics and added atmospheric vocals. I would listen to Sopor Aeternus more but I am freaked out by that Anna-Varney person.
--Symptom of the Universe - Metallica
You should always praise someone who happens to interpret his favorite song. It's Metallica (you may have your opinion of them, I don't even listen to them anymore) paying tribute to the Masters of the genre.
13 Sabbath Jam - Eyehategod
Finally, someone understands that Cornucopia has a killer intro, but is a lackluster song. So they take the intro off it in order to introduce Hand of Doom and end with Behind the Wall of Sleep. A great amalgamation of three great songs.
12 Black Sabbath Medley - Candlemass
Why only cover one when you can cover seven for the same price? Begin from Symptom of the Universe, then turn on Sweet Leaf, follow right through Sabbath Bloody Sabbath, use Into the Void to enter to Electric Funeral and finish with Supernaut and the deep riff that started it all from Black Sabbath. Doom doesn't get more epic than this.
11 Iron Man / Sabbath Bloody Sabbath - The Cardigans
What? You dare turn Metal into Britpop??? A largely silent and soothing ballad that tells the story of the metal monster from the future, that's trippy, man!!!
10 Black Sabbath - Amber Asylum
What? You dare turn Metal into Neoclassical? Amber Asylum creates gloomy ambients that would easily fit into funeral doom, but retain that sweetness of spring rain.
9 Heaven and Hell - Solitude Aeturnus
What? You dare cover a song from the Dio years? Rob Lowe demonstrates he belongs to the great voices of Metal alongside Ronnie James Dio. They also make a killer job covering Ozzy Osbourne's No more Tears.
8 Behind The Wall Of Sleep - Static X
Not the best cover in the world, but I admit I have a liking for Static X's brand of brash, young and energetic metal that does not fall into Metalcore.
7 Black Sabbath - Wraith of the Ropes
Again, take the killer intro for Cornucopia to create a dirty and heavy cover of the original Doom masterpiece and lace it with horror movie undertones. The piano takes a lot from John Carpenter's scores.
6 Black Sabbath (From the Satanic Perspective) - Type O Negative
Change the lyrics, but remain true to the original, they decided to be the Devil instead of the victim. Don't you find it funny that the original, which spawned many interesting things, says "Could this be the end?". Instead, they say; "This is just the start!"
5 Snowblind - Evoken
Masters of Funeral Doom Evoken bring their gargantuan weight and special brand of darkness to the original Stoner Doom masterpiece.
4War Pigs - Ozzy Osbourne
Not a cover but a correction, back in the day they could not sing about Black Magic so they renamed their song "Walpurgis" into something more political (early 70's zeitgeist, after all). This is the song restored to its original glory and the lyrics for my taste are much, much better.
3 Hand of Doom - Slayer
They change the coda to something more fitting of a heroin trip, instead of finishing with a whimper they go on an adrenalin stroke.
2 Solitude - Ulver
This version of Solitude is more silent and intimate, more at home in the Shadows of the Sun album. and let's face it; Kristoffer Rygg's voice is much better for the song than Ozzy's
1 Zero the Hero - Cannibal Corpse
No one covers the Ian Gillan era of Sabbath!!!! But this cover is so surprising, the fade-in intro creates an abbatoir-like ambient that Corpsegrinder's vocals are more at home than the original.